A thermal barrier coating system may be used to protect the components of a gas turbine engine that are subjected to the highest material temperatures. The thermal barrier coating system usually includes a bond coat that is deposited upon a superalloy substrate, and a ceramic thermal barrier coating that is deposited upon the bond coat. The thermal barrier coating acts as a thermal insulator against the heat of the hot combustion gas. The bond coat bonds the thermal barrier coating to the substrate and also inhibits oxidation and corrosion of the substrate.
The currently preferred thermal barrier coating is yttria-stabilized zirconia (YSZ), which is zirconia (zirconium oxide) with from about 3 to about 12 percent by weight yttria (yttrium oxide) added to stabilize the zirconia against phase changes that otherwise occur as the thermal barrier coating is heated and cooled during fabrication and service. The YSZ is deposited by a physical vapor deposition process such as electron beam physical vapor deposition. In this deposition process, the grains of the YSZ form as columns extending generally outwardly from and perpendicular to the substrate and the bond coat.
When the YSZ is initially deposited, there are small gaps desirably present between the columnar grains. The gaps serve to accommodate the transverse thermal expansion strains of the columnar grains and also act as an air barrier in the insulator structure. As the YSZ is cycled to elevated temperature during service, these gaps gradually close by a sintering mechanism. As a result, the ability of the YSZ to accommodate thermal expansion strains gradually is reduced, and the thermal conductivity of the YSZ gradually increases by about 20 percent or more. The thickness of the YSZ must initially be greater than would otherwise be desired, to account for the loss of insulating power associated with the rise in thermal conductivity during service.
It has been recognized that the addition of sintering inhibitors to the YSZ reduces the tendency of the gaps between the columnar grains to close by sintering during service of the thermal barrier coating. A number of sintering inhibitors have been proposed. However, these sintering inhibitors have various shortcomings, and there is a need for more effective sintering inhibitors. The present invention fulfills this need, and further provides related advantages.